The present invention relates to a method for the treatment of a material web. The invention also relates to an apparatus for the treatment of a material web.
In prior art, various solutions for the treatment of a material web in a paper machine or the like are known. Commonly known is e.g. a solution in which, to help release the paper web from the middle roller in the press section of the paper machine and from the roller of the 4th press, a releasing chemical is supplied into the jet lubricating the doctor element scraping the roller surface. This is done in the hope that some of the chemical will pass between the doctor and the roller and form a film on the roller surface. The purpose of the film formed by the chemical is to facilitate the release of the web from the roller surface e.g. by reducing or increasing the surface tension of water, depending on the composition of the chemical.
The prior-art solution has certain drawbacks. When the doctor element is working properly, it scrapes all water, dirt and possible chemicals away from the roller surface so that the roller surface after the doctor is dry and no film is left on it. The efficiency of the chemicals remains low and large amounts of them are wasted. On the other hand, if a larger amount of chemical is passed through between the doctor and the roller, this means that the scraping effect is deteriorated, which again results in web breaks and a loss in production. In prior-art supply methods, the efficiency of the chemicals remains low and the consumption of chemicals is often too high, and still no sufficient effect on the release of the paper web is achieved.
The object of the present invention is to achieve a completely new type of solution to various problems regarding the release and runnability of the material web in a paper or cardboard machine or a similar apparatus. The invention is based on a procedure in which the behavior of the material web is influenced by using a gas, such as compressed air or an equivalent medium, a particularly preferred medium being dry air. The gas is dispensed via a special gas dispensing means/device and/or element to the material web or to its immediate vicinity.
In more precise terms, the invention is characterized by what is presented in the claims.
The solution of the invention has several significant advantages. The method of the invention for dispensing gas, especially dry air, can be used in numerous applications. It can be used e.g. for improvement and control of the release of a material web, such as a paper web, from the rollers. Fluctuations occurring in the release of the web can be managed more effectively than before, thus allowing easier and more reliable release of the paper web from the roller. Therefore, only a minimal tension is needed for the release. This means that the strength properties of the paper web are not impaired as the web need not be stretched in order to release it. This also reduces the proneness of the paper machine to web breaks and increases its speed potential.
In addition, web movement e.g. in the open transfers of the paper web in the press section of the paper machine can be improved and possible problems regarding rucking up of the edges of the paper web and formation of so-called air bells in the paper web can be reduced. Moreover, the solution of the invention can be used to control and reduce the problems of slackness of the edge areas of the paper web. The above-described management and reduction of the problems of rucking up and slackness of the edges of the paper web improve the runnability of the paper machine while at the same time allowing the high paper web tension due to the above-mentioned problems to be reduced, which again reduces the proneness of the paper machine to web breaks and increases its speed potential.
In addition, the gas dispensing system of the invention can be used to influence the properties of the paper web, e.g. the uniformity of its moisture profile and dry matter content for example in the press section of the paper machine, by reducing the moistening effect on the paper web of the water jet at the doctor element on the center roll of the paper machine and increasing the temperature of the paper web by using e.g. dry heated air as the gas dispensed. The increased dry matter content of the paper web after the press section has a very great economic importance because the drying steam constitutes one of the largest individual expense items in paper manufacture.